The conventional methods of separating various hydrocarbons involves technology which includes extraction, absorption, distillation, etc. techniques. However, a disadvantage of utilizing these techniques involves the use of energy which, in the present time, is relatively expensive. For example, in one aspect of petroleum refining a particular application step involves the deasphalting of a heavy crude oil which is recovered from a petroleum source such as a well. The crude oil is generally first passed to an atmospheric pipe still to produce light gases as well as a variety of distillation cuts suitable for use in the synthesis of fuels such as gasoline, etc. as well as cuts which may be useful in the petrochemical industry. The bottoms or bottom fraction from this atmospheric distillation still may then be passed to a vacuum distillation still which will recover lighter products as volatile material which can then be utilized to prepare other fuels such as diesel oil. However, as the feedstocks become heavier in nature, a significant amount of the fuel oil is not volatilized in either the atmospheric or vacuum stills.
In order to recover still useable oil fractions, the heavy bottoms from the vacuum distillation still is then subjected to the action of a solvent which comprises a relatively light hydrocarbon such as a paraffinic hydrocarbon containing from 3 to 7 carbon atoms. In order to obtain the desired fractionation of useable products, a high ratio of solvent to oil is required. For example, in the case of asphaltenes which are precipitated out of the heavy fraction, a typical process will operate with a solvent to oil weight ratio of 4:1 to 10:1 depending upon the degree of oil-asphaltene separation desired. The desired oil fraction, after separation from the precipitated asphaltenes will be recovered while, due to the expense of the solvent required for the separation, it is necessary to recover the solvent for further use. The high ratio of solvent to oil usually requires energy intensive processes due to the need for recovering the solvent by volatilization.
In view of the expense associated with the volatilization process, it is deemed necessary to provide a low-cost method of separating preferred light oils from heavy oils which contain a high asphaltene content. In addition, many crude oils also contain metal impurities which have a deleterious effect on the use of light oils for various purposes such as, for example, fuel for internal combustion engines, etc. Therefore, these metals must also be removed from the oils prior to their use thereof. It has now been discovered that a method for separating relatively light hydrocarbons or oils from relatively heavy oils and metals may be accomplished by passing a solution of heavy oils and a hydrocarbon solvent through a membrane of the type hereinafter set forth in greater detail whereby an effective separation of light oils from heavy oils and unwanted metals may be effected.
U.S. Pat. No. 3,305,595 is drawn to a process for the dialysis of heavy aromatic oils. However, the process described in this patent differs from the process of the present invention. Dialysis is the separation of solutes by means of the unequal diffusion of these solutes through membranes or diaphragms. The membrane is a continuous phase and the separation occurs by the components dissolving in the membrane phase and diffusing to the other side. In dialysis, the object is to remove solutes of certain molecular weights by taking advantage of the fact that these solutes possess a higher permeation coefficient in the membrane material than do solutes which possess other molecular weights. The bulk flow of the solvent through the membrane is prevented by balancing the osmotic pressure of the feed solution by using a flowing isotonic, that is, the same osmotic pressure solution on the other side of the membrane to take up the solutes passing through the membrane.
In contradistinction to the dialysis process, the process of the present invention utilizes a separation process which operates by ultrafiltration. In ultrafiltration, relatively large molecules are concentrated in solution by removing a portion of the solvent. Pressure is used to drive the solvent through membranes, the pressure difference typically being from about 10 to about 100 pounds per square inch gauge, in contrast to the dialysis process which, as hereinbefore set forth, is based on a diffusion-controlled rate.
U.S. Pat. No. 3,919,075 discloses a process for regenerating used lubricating oils by means of an ultrafiltration membrane. The patent teaches that a solution may be combined with the used lubricating oil to assist in the regeneration of the oil. However, both paraffinic solvents and aromatic hydrocarbons are disclosed as being suitable for use in the regeneration process. As will hereinafter be shown in greater detail, it is impossible to utilize the process of the present invention utilizing aromatic hydrocarbons as solvents for the process. The patent to Parc et al. does not recognize that the treatment of a multicomponent feedstock such as a heavy hydrocarbon crude oil necessitates the use of a specific class of solvents which have been found to be particularly efficient in serving both as a solvent for the crude oil as well as avoiding any deleterious affect upon the membrane which is employed to effect the separation process.
As will hereinafter be shown in greater detail it has now been discovered that by utilizing a particular type of solvent it is possible to combine the two advantages of solubilizing the heavy crude oil without damaging the membrane and thus permitting the membrane to maintain its integrity in the presence of the solution of oil dissolved in a solvent.